The maintenance of genome integrity is essential for the health of individuals and for faithful propagation of genetic material to future generations. The Mrel 1/Rad50/NBS1 (MRN complex) is thought to be a global player in the double strand DNA repair pathway. Mutations in Mrel 1 protein lead to ataxia-telangiectasia like disorder in humans and somatic mutations of Mrel 1 have been observed in cancers. The MRN complex is an early respondent to breaks possibly holding the strands together and signaling ataxia telangiectasia mutated (ATM) kinase. However, Mrel 1 also possesses nuclease activities for which in vivo roles are not well understood. In order to gain better understanding of the role of Mrel 1 and its nuclease function our lab constructed three novel targeted alleles in mouse;an allele with a H129N mutation causing nuclease deficiency only, a wild type conditional allele, and a null allele causing MRN deficiency. Previously published work by the applicant using these alleles demonstrated that the nuclease activities of Mrel 1 are essential for embryonic viability and genome maintenance. It is the aim of this proposal to further determine the roles of Mrel 1 in the various aspects of DNA repair, genome stability and cancer formation in mammals. Specific aims are to: 1. Test the hypothesis that mammalian Mre11 endonuclease and exonuclease activities have distinct in vivo functions. 2. Determine roles of the Mrel 1 protein in somatic tissues. 3. Test the hypothesis that Ku and Mrel 1 are linked in the mammalian DNA damage response. It was initially determined that nuclease deficiency causes early embryonic lethality. Using previously prepared Mre11 deficient embryonic fibroblasts, we will express various endo or exonuclease deficient mutans of Mre11 and test for genomic instablity. In order to test for roles of Mrel 1 in cancer we will cross our mouse alleles with Mx-Cre to delete Mrel 1 from whole mouse tissues and then examine the effects of this on different organ systems To test for Ku rescue of Mrel 1 deficiency we will cross our Mrel 1 mouse alleles with Ku deficient mice and determine if the resulting double mutant mice, embryos or cell lines are more viable. Mre11 is a human disease gene thought to function in DNA repair and the maintenance of the human genome. Genome maintenance is crucial to cancer prevention. This proposal aims to study the role of its unknown nuclease function in relation to its the cellular response to DNA damage.